Liquid foams of reinforced unsaturated polyester resins and process for making the same

ABSTRACT

A process for obtaining fiber-reinforced foams suitable to be transformed to reinforced solid cellular materials having a density lower than 0.7 kg/l, if inert fillers are not present, and lower than 1 kg/l if inert fillers are present as well, essentially by mechanical incorporation of gas or a mixture of gases into an unsaturated polyester resin containing conventional additives is described. As reinforcing materials synthetic, vegetable or mineral fibers having a maximum length of 3 mm are employed. The mixture of the resin with the fibers, and optionally with the inert fillers, is conveyed by at least one pump to a mechanical foaming device which introduces into the mixture gas in form of small bubbles, homogeneously and uniformly distributed therein.

This is a continuation-in-part of application Ser. No. 145,251, filedApr. 30, 1980.

BACKGROUND OF THE INVENTION

The present invention relates to fiber-reinforced liquid foams made fromunsaturated polyester resins, suitable to be transformed into reinforcedsolid cellular materials having a density lower than 0.7 kg/l if saidmaterials contain reinforcing fibers but not fillers, and lower than 1.0kg/l, if inert fillers are also present.

The present invention also relates to a process for preparing saidliquid foams essentially by mechanical incorporation of a gas or amixture of gases into an unsaturated polyester resin reinforced withsynthetic, vegetable or mineral fibers, containing conventionaladditives.

Further objects of the present invention will be described hereinafter.

Solutions of unsaturated polyester resins in styrene, reinforced withthe reinforcing materials hereinbefore specified, are very difficult totransform into foams, inasmuch as only with difficulty do such solutionsretain air or inert gases occluded in a stable and regular manner.

In order to overcome this drawback it has been proposed to employchemical foaming agents such as e.g. isocyanates, certain azo-compounds,and so forth, which are added to the unsaturated polyester resin at themoment at which the foams are to be formed.

In practice, however, said agents have not yielded satisfactory results,because the copolymerization temperature of the solutions of unsaturatedpolyester resins in styrene begins to rise very slowly, so that theresin gels a long time before the temperature has become high enough tocompletely activate the foaming agent.

Further said processes have other disadvantages, such as e.g.:

the chemical agents employed as foaming agents are usually toxic and aregenerally unstable at room temperature and therefore must be maintainedat low temperatures until they are used; and the process is noteconomical inasmuch as the chemical foaming agent is expensive.

It has also been proposed (U.S. Pat. Nos. 4,120,923 or 3,362,919) todissolve, under pressure, in the unsaturated polyester resin, gases suchas carbon dioxide or air, which, once the pressure is released, forexample by means of a nozzle, will cause the resin to foam.

However this type of foaming known as "physical foaming" is not suitablefor continuous industrial processes. If reinforcing fibers are presentin the resin to be foamed, than the nozzle will become quickly occludedwith such fibers and no foam will be produced.

According to another discontinuous method of physical foaming, volatile,low boiling, halogenated hydrocarbons such as CClF₃, CHClF₂ or CCl₂ F₂are dissolved in the resin at a low temperature and foam is producedwhen the temperature is raised above the boiling point of thehydrocarbon.

The foam eventually produced by these methods has large gas bubbleswhich are not homogeneously distributed.

SUMMARY OF THE INVENTION

The inventor has now surprisingly found a process whereby it is possibleto obtain, essentially by mechanical introduction of gas, a liquidfiber-reinforced unsaturated polyester foam, suitable to be transformedinto solid cellular materials reinforced with synthetic, vegetable ormineral fibers having a density lower than 0.7 kg/l, if inert fillersare not present, or a density lower than 1 kg/l when inert fillers arepresent as well.

An object of the present invention is therefore a continuous process formaking liquid fiber-reinforced foams suitable to be transformed intoreinforced cellular solid materials having a density lower than 0.7 kg/lin the case of reinforced materials not containing inert fillers or adensity lower than 1.0 kg/l if inert fillers are present as well,essentially by mechanical incorporation of a gas or a mixture of gasesinto an unsaturated polyester resin which contains conventionaladditives, characterized by the fact that synthetic, vegetable ormineral fibers having a maximum length of 3 mm, and preferably less than1.5 mm, are used as reinforcing materials, and that the mixture of saidresin with said fibers is conveyed by at least one pump to a mechanicalfoaming device suitable to introduce gas in the form of small bubbleshomogeneously and uniformly distributed in the aforesaid mixture.

Preferably the diameter of the majority of the bubbles does not exceed0.3 mm.

The percentage of the reinforcing material is suitably maintainedbetween 10 and 30% by weight with respect to the unsaturated polyesterresin. Glass fibers having the aforesaid length are preferably employedas reinforcing materials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When glass fibers are used as reinforcing materials, said fibers have adiameter lower than 16 micron; if synthetic, vegetable or mineral fiberssuch as e.g. aramide, acrylic, carbon cellulose ester,polyethyleneterephthalate, regenerated cellulose fibers, are employed asreinforcing material, said fibers have a count lower than 20 deniers.

As has already been recognized in the art, the addition to unsaturatedpolyester resin compositions of inert fillers, such as calciumcarbonate, or alumina hydrate in the form of finely divided powders, inan amount up to 40% by weight with respect to the overall weight of themixture, has the effect of improving some particular properties of thecellular materials, e.g. reduces the product costs, increases theelastic modulus, lowers the shrinkage during cross-linking and mayconfer to the cellular material self-extinguishing properties.

According to the present invention, a turbine, formed by a stator and arotor, provided with prismatic or cylindrical projections having aperipheral speed from 200 to 500 m/min is essentially employed as themechanical foaming device suitable to introduce the gas into theunsaturated additive containing polyester resin.

According to the present invention the apparatus for the production ofreinforced foams preferably comprises a stirrer-mixer in which thesolution of unsaturated polyester alkyds in styrene containing suitableadditives is mixed with the reinforcing fiber material hereinbeforespecified (preferably milled glass fibers or chopped strands up to alength of 1.5 mm) and is then introduced through a feed pump into theturbine which has the aforesaid characteristics.

Said turbine is conveniently heated by means of heating jackets providedboth on the stator and the rotor of the turbine.

The structure of the turbine preferred for the foaming process accordingto the present invention is illustrated in the exploded view of theattached FIGURE. The dimensions of this turbine may vary in a wideinterval depending on the amount of foam which is desired to be producedper unit of time. From the economical point of view, however, it isdesirable to select, for industrial uses, turbines having a freeinternal volume not less than 2-3 liters which are suitable for foamingfrom 10 to 100 liters/hour of mixture fed (comprising resin, additives,reinforcing fibers and optionally fillers). Within the limits indicated,the amount of foam produced in the turbine will be determined andcontrolled by the flow rate of the feeding pump. If higher amounts offoam are desired to be produced, it is necessary to use turbines havinglarger free volumes and therefore capable to foam higher amounts offeed. There are no critical upper limits for the turbine dimensions,except for economical considerations related to the hourly amount offoam which may be consumed in the production of cellular materials.

An example of a turbine suitable for industrial production onsmall-medium scale has the following dimensions

free volume of the turbine=4 liters

length of the stator=26.3 cm

diameter of the stator=10 cm

length of the rotor=20 cm

position of projections (pegs)=in alternate rows on the stator and therotor

foaming capacity=adjustable between 30 and 200 liter/hour of mixturefed.

For industrial production on medium-large scale a turbine can be usedwhich has the following characteristics: (turbine "A")

free volume of the turbine=14 liters

length of the stator=41.5 cm

diameter of the stator=31 cm

length of the rotor=31.6 cm

position of pegs=in alternative rows on the stator and the rotor

foaming capacity=adjustable between 100 and 800 l/h of mixture fed

density of the foam produced=adjustable by varying the ratio between theamount of mixture fed and the amount of gas introduced per unit of time.

In the examples which will follow illustrating the process of thepresent invention turbine "A" was employed.

In said turbine a gas (preferably nitrogen or air) which constitutes thefoaming element is also introduced under an appropriate pressure ofbetween 1.1 and 10 absolute atmospheres, preferably between 2 and 4absolute atmospheres. In the last or last but one row of projections ofthe turbine, where the foam has already been created, the catalyst isintroduced so that the foam thus produced is already catalyzed when itflows out of the turbine.

The liquid foams, reinforced with synthetic, vegetable or mineral fibershaving a length less than 3 mm, as hereinbefore specified, form afurther object of the present invention.

The cross-linking of said foams and thus the shaping of the desiredarticles, are carried out in conventional devices such as e.g. in openmoulds or in continuous shaping devices. The reinforced solid cellularmaterials, which have preferably more than 80% closed cells, obtained bycross-linking the above specified liquid foams, e.g. in open moulds orin continuous shaping devices, also form an object of the presentinvention.

The expression "unsaturated polyester resins" is to be construed,according to the present invention, as meaning the resins obtained froman unsaturated polyester alkyd, formed by polycondensation of at leastone-α-β-ethylenically unsaturated dicarboxylic acid and/or its anhydridewith at least one polyvalent alcohol and by dissolving the thus obtainedunsaturated polyester alkyd in one or more ethylenically unsaturatedmonomers, such as e.g. styrene,α-methylstyrene, methyl methacrylate,diallylphthalate, vinyl toluene, etc.

Examples of ethylenically unsaturated dicarboxylic acids or thecorresponding anhydrides, comprise maleic acid or its anhydride,fumaric, itaconic and mesaconic acids. Together with the unsaturateddicarboxylic acids there may be employed saturated, mono orpolyfunctional aliphatic carboxylic acids, such as adipic, succinic,glutaric acid and the like; aromatic, mono or polyfunctional carboxylicacids, such as phthalic, isophthalic, terephthalic, benzoic acid, etc.,anhydrides such as phthalic, trimellitic anhydride, etc.

As polyvalent alcohols there may be employed:

ethylene glycol, propylene glycol, 1,2-butanediol, diethylene glycol,dipropylene glycol, 1,6-hexanediol, 1,4-cyclohexane methane diol,trimethylol propane, pentaerythrite, glycerine, neopentyl glycol, etc.

The usanturated polyester resins may be prepared by the known processesused for the polyester resins, both in solvent and in dry phase.

According to a practical and preferred embodiment of the presentinvention, the unsaturated polyester resin is obtained by reacting from1 to 1.3 mols of at least one polyvalent alcohol with from 0.05 to 1 molof at least one ethylenically unsaturated dicarboxylic acid and/or itscorresponding anhydride. The polycondensation is continued until apolymer is obtained having an acid number of between 5 and 90 mg ofKOH/g. The preferred acid number is of between 10 and 30 mg of KOH/g andthe molecular weight between 500 and 5000.

Besides the ethylenically unsaturated dicarboxylic acid and/oranhydride, at least one saturated dicarboxylic acid in an amountcomprised between 5 and 95% (molar percentage) with respect to theunsaturated dicarboxylic acid may also be present in the mixture,according to the known art.

The unsaturated polyester thus obtained is mixed with styrene, in apolyester/styrene ratio comprised between 9:1 and 1:1, preferably 4:1and 2.5:1.

Inhibitors and stabilizers suitable to prevent the prematurecross-linking of the mixture may be added to the styrene/unsaturatedpolyester mixture in amounts from 0.1 ppm to 10000 ppm.

The most commonly used inhibitors and stabilizers are: hydroquinone,quinone, quinhydrone, tertiarybutylpyrocatechol, toluenehydroquinone,monotertiary butylhydroquinone, ditertiary butylhydroquinone,1,4-naphthoquinone, anthraquinone, methyl and/or ethyl esters ofhydroquinone, picric acid, trinitrobenzene, paraphenylenediamine, etc.

Further to improve the stability of the resin, organic or inorganiccompounds soluble in the polyester, quaternary ammonium salts, etc. areadded.

The reinforcing material according to the invention is added to theunsaturated polyester resin thus obtained, and then the whole is mixedwith a gas under continuous stirring in the turbine hereinbeforedescribed, wherein a cross-linking catalyst system of a known type isalso introduced. A reinforced and homogeneous foam is thus obtainedwhich may be poured into open molds and allowed to stay underatmospheric conditions for a period of time sufficient to obtain thecomplete cross-linking, whereby a shaped rigid cellular material isformed.

In order to improve the stability of the foam, additives are added tothe unsaturated polyester resin before foaming, such as e.g. surfaceacting agents for promoting the formation of the foam and/or agents forregulating the diameter of the gas bubbles and/or foam stabilizingagents, e.g. surface active agents based on silicone compounds, blockcopolymers of silicones with polyethers, soaps such as ricinoleates,polymercaptanes, etc.

A catalyst system and any compound or mixture of compounds whichgenerate free radicals under the particular conditions of polymerizationmay be used.

These compounds are: the peroxides or the hydroperoxides, such asdiacetyl peroxide, benzoyl peroxide, hydrogen peroxide, cumenehydroperoxide, methylethylketone peroxide, etc. Other catalysts whichgenerate free radicals may also be used, such as e.g. ammoniumpersulphate, perborates and percarbonates.

In addition to the free radical generating catalyst it is preferred toemploy also an accelerator which increases the speed of decomposition ofthe peroxidic compound, which thus produces free radicals at higherspeeds. As an accelerator, cobalt naphthenate is generally employed,which is diluted with styrene until it is brought to a concentration ofabout 1-3% of metal. A complexing agent for increasing the efficency ofthe accelerator may also be used.

According to a widely accepted interpretation, it is believed that theseagents act to reduce the cobalt naphthenate which passes to thecorresponding cobaltous compound which is much more reactive.

Generally aromatic tertiary amines, preferably dimethylaniline are usedas complexing agents.

Broad variations and changes may be effected in carrying into practicethe present invention, without departing from the spirit and the scopeof the present invention.

In order better to illustrate the inventive idea of the presentinvention and to carry the same into practice, the followingnon-limitative example is described.

EXAMPLE 1

In a steel vessel, there are charged 100 kg of a polyester resinprepared in a reactor by mixing the following compounds according to thefollowing molar ratios: propylene glycol 0.8 mols, diethylene glycol 0.2mols, maleic anhydride 0.6 mols, phthalic anhydride 0.4 mols,hydroquinone 100 ppm, styrene 30% with respect to the resin. To theaforesaid resin are added 500 ml of cobalt octoate in xylene at aconcentration of 6%, 1 kg of silicone oil, 15 kg of milled glass-fibershaving a nominal length of 0.8 mm. The whole is mixed with an helicalstirrer for about 20 minutes. The aforesaid mixture is transferred fromthe vessel, by using a gear pump having a flow rate of 200 l/h, into aturbine and thermostatized at 35° C. The aforesaid turbine isconstituted by a stator and a rotor both provided with cylindricalprojections.

The turbine, as indicated previously for turbine "A", has the followingdimensions:

free volume=14 liters

length of stator=41.5 cm

diameter of stator=31 cm

length of the rotor=31.6 cm

position of projections=in alternative rows on the stator and on therotor

foaming capacity=adjustable, between 100 and 800 l/h of mixture fed.

The peripheral speed of the rotor is 230 m/min. The foaming gas (air)having a flow rate of 360 Nl/h (normal lt/h)anf at a pressure of 2.5absolute atmospheres, is injected into the turbine from a nozzle.Methylethylketoneperoxide is introduced with a flow rate of 2 l/h at thelevel of the last but one row of projections before the foam leaves theturbine.

The pressure existing in the turbine permits the foam to flow out with aflow rate of about 560 l/h in such a way that it may be poured into amould at 35° C. Since the polymerization time is about 10 minutes, thefoam solidifies. The following physical and mechanical characteristicsare measured on samples obtained from the cellular material thusproduced:

density: 0.45 kg/dm³

Tensile strength: 47 kg/cm²

Tensile modulus: 9780 kg/cm²

Elongation at break: 0.74%

Compression strength

at 7% deformation: 78 kg/cm²

shear strength: 27 kg/cm²

shear modulus: 623 kg/cm²

Minimum falling height of a steel

ball weighing 5 kg which causes the breakage of a sample having athickness of 1 cm-120 cm

EXAMPLE 2

The same resin of Example 1 is used, containing the same additives inthe same amounts and the same amounts of glass fibers having the samecharacteristics.

To this mixture 50 kg of calcium carbonate are added in powder form.

The whole is mixed by means of a helical stirrer for about 20 minutes.The aforesaid mixture is transferred from the vessel into the turbine ofExample 1 by means of a gear pump and is thermostatized at 35° C. Theperipheral speed of the rotor is 230 m/min.

From a nozzle nitrogen is injected into the turbine at a flow rate of360 Nl/h (normal l/h) and a pressure of 2.5 absolute atmospheres.Methylethylketoneperoxide is introduced into the turbine with a flowrate of 2 lt/h at the level of the last but one row of projectionsbefore the foam leaves the turbine.

The pressure existing in the turbine permits the foam to flow out, witha flow rate of about 560 l/h and a density of about 0.7 kg/l. The foam,poured into a mold heated to 35° C., cross-links in about 10 minutes,whereby a solid cellular material having a density of about 0.8 kg/l isobtained. Its elastic modulus is 12500 kg/cm².

EXAMPLE 3 (Comparison example)

This example has the purpose of demonstrating the criticallity of thelength of the glass fibers.

The same resin, additives and glass fibers mixture of Example 1 is usedexcept that the glass fibers have a length of 35 mm (1.4") instead of0.8 mm. In order to mix the ingredients the same vessel and helicalstirrer of Example 1 are used.

The aforesaid mixture is fed, by means of the same gear pump of Example1, again with a flow rate of 200 l/h into the turbine "A" of Example 1,thermostatized at 35° C., the peripheral speed of the rotor being 230m/min.

From the nozzle air is injected into the turbine with a flow rate of 360normal l/h and a pressure of 2.5 absolute atmospheres. Methylethylketoneperoxide is added with a flow rate of 2 liters/hour at the last but onerow of projections.

It is observed that the gear pump reduces rapidly its flow rate becauseof glass fibers deposition, while the turbine stops working after a fewseconds due to occlusion.

Initially a resin containing a few gas bubbles and a few glass fibers,which is not foamed flows out of the turbine. Successively, however,only pure, not-foamed resin comes out.

The process of Example 3 is therefore not suitable for producing a foamreinforced with glass fibers.

EXAMPLE 4 (comparison example)

This example has the same purpose of Example 3.

Example 3 is repeated except that the glass fibers employed have anaverage length of 6.5 mm.

As in the case of Example 1, a reduction of the flow rate of the gearpump is observed because of the deposition of glass fibers, after a fewminutes. From the turbine, initially, a foam flows out, having a densityof about 1 kg/l and containing relatively few gas bubbles and only veryfew glass fibers. After a few minutes, the turbine becomes blockedbecause of occlusion due to the deposition of glass fibers and onlyresin comes out which is not foamed. The conclusions are the same as inExample 3.

EXAMPLE 5 (comparison example)

This example has the purpose of illustrating the results obtained usingan unsaturated polyester resin reinforced with glass fibers, accordingto the present invention, but foamed by means of a gas dissolved in theresin (physical foaming) instead of by mechanical introduction of thegas.

The same resin and additives of Example 1 are used.

By means of a helical stirrer the following components are mixed:

10 kg of the resin of the resin of Example 1 containing the sameadditives (hydroquinone, cobalt octoate, silicone oil) of Example 1 and1.5 kg of glass fibers having a nominal length of 0.8 mm.

The mixture is transferred to an autoclave and 86 cm³ ofmethylethylketone peroxide are added to it. The autoclave is rapidlyclosed and, through a tube immersed into the mixture, gaseous carbondioxide is introduced at a pressure of 5 atmospheres up to saturation.Under the constant pressure of 5 atmospheres, the mixture is ejectedthrough a spinneret the orifices of which have a diameter of 0.3 mm.Because of the decompression occuring at the exit from the spinneret,the gas dissolved in the resin is liberated in the form of bubblescausing the mixture to foam. However, the orifices of the spinneretbecome very rapidly occluded with glass fibers and therefore thespinneret becomes blocked in less than one minute and the device causesto produce foam. The same phenomenous occurs again after having cleanedthe orifices of the spinneret.

The method of physical foaming by dissolution of a gas is thereforeenable to produce foams reinforced with glass fibers and is not suitablefor a continuous industrial process.

EXAMPLE 6 (comparison example)

This example has the purpose of illustrating the results obtained byusing an unsaturated polyester resin reinforced with glass fibersaccording to the present invention but foamed by means of a low boilingliquid dissolved in the resin (physical foaming).

The same resin and the same additives and glass fibers are used as inExample 5, except that these components are mixed at a temperaturecomprised between 10 and 15° C. in order to avoid any possible prematurevolatilization of the blowing agent.

To 10 kg of resin containing the additives specified in Example 1, 1.5kg of glass fibers having a nominal length of 0.8 mm, 86 cm³ ofmethylethylketone peroxide and 12.5 g of monofluorotrichloromethane(known as FREON 11) are added.

The mixture thus obtained is pumped into a closed mold having athickness of 4 cm, provided with discharge holes and pre-heated to 50°C. The amount of mixture introduced into the mold corresponds to about30% of the internal volume of the mold.

After a setting (hardening) time of 30 minutes at 50° C., the mold isopened and the article produced is extracted from it. The product has adensity of 0.45 kg/lt.

Comparing the mechanical properties of this product to those of theproduct of example 1 it is observed that the tensile modulus of theproduct obtained in example 1 is 15-20% higher than the correspondingvalues of the product obtained according to example 6.

EXAMPLE 7

(Comparison of the properties of the cellular materials obtainedaccording to the present invention to those obtained according to theprior art).

From the cellular materials obtained according to examples 1 and 6specimen are cut along the thickness of the articles which is of 4 cm.These specimen are examined by means of an optical microscope atmagnification 20.

The gas bubbles contained in the specimen obtained from the materialobtained in Example 1 are observed to be homogeneously distributedthroughout the tickness to five equal layers, it is noted that eachlayer containes 20±1% of the total amount of the bubbles contained inthat thickness.

Examining in the same manner the specimen cut from the article obtainedin Example 6 an unhomogeneous distribution of the gas bubbles isobserved throughout the thickness of the article. By dividinggraphically said thickness into five layers of equal thickness it isnoted that the central layer contains from 30 to 40% of the total volumeof the bubbles while the two PG,25 external layers contain, each, lessthan 10% of the total volume of the bubbles.

Measuring, again by means of the optical microscope, the dimensions ofthe gas bubbles, it is found that, in the case of the specimen obtainedfrom the material of Example 1, 75% of the gas bubbles have a diameterof between 0.1 and 0.3 mm, 15% of the bubbles have lower diameter and10% have diameters higher than 0.3 mm.

The same measurement effected on the specimen of Example 6 shows a muchless homogeneous distribution of the bubble dimensions. In this caseonly about 50% of the bubbles have diameters of between 0.1 and 0.3 mmwhile about 35% have lower and 15% have higher diameter.

These two types of nonhomogeneity will negatively influence themechanical properties of the cellular material, as indicated in Example6.

I claim:
 1. A continuous process for making fiber-reinforced foams whichare suitable for transforming into reinforced solid cellular materialshaving a density lower than 0.7 kg/l if no inert filler material ispresent or a density lower than 1 kg/l if inert fillers are present aswell, said process essentially comprising:(1) mixing together anunsaturated polyester resin, a reinforcing material and, optionally aninert filler, said reinforcing material being selected from the groupconsisting of synthetic fibers, vegetable fibers and mineral fibershaving a maximum length of 3 mm, whereby a liquid mixture is obtained;(2) transferring said liquid mixture of reinforcing material,unsaturated polyester resin and, optionally, said inert filler to aturbine having a free internal volume of at least 2 liters andcomprising a rotor and a stator, both having either prismatic orcylindrical projections, said turbine rotor having a peripheral speed ofgreater than 200 m/min and; thereafter (3) introducing a gas into saidturbine at a pressure of from 1.1 to 10 absolute atmospheres, saidturbine operating to distribute said gas in the form of small bubbleshomogeneously and uniformly throughout said mixture, thereby producing areinforced foam of unsaturated polyester resin, and thereafterdischarging said reinforced foam from said turbine.
 2. A processaccording to claim 1, wherein said turbine rotor has a peripheral speedof 200 to 500 m per minute.
 3. A process according to either claim 1,wherein said reinforcing material is preeent in an amount of from about10 to 30% by weight based on the amount of unsaturated polyester resin.4. A process according to claim 1, wherein said reinforcing material isglass fibers having a diameter of less than 16 microns.
 5. A processaccording to claim 1, wherein said inert filler is selected from thegroup consisting of calcium carbonate and alumina hydrate and whereinsaid inert filler is added in the form of finely divided powder in anamount of up to 40% by weight with respect to the overall weight of themixture.
 6. A process according to claim 1, wherein said gas isintroduced into said turbine at a pressure comprised between 2 and 4absolute atmospheres.
 7. A process according to claim 1, wherein thegreater part of said bubbles have a diameter not substantially higherthan 0.3 mm.